According to this week Autosprint the problem with Ferrari tunnel is that it "lost calibration" over time.

I have been trying to imagine what could cause that.
Assuming that the instruments are regularly calibrated and worn parts replaced, just creeping-sagging of the walls themselves comes to mind, but it seems quite a strong effect for displacements which are presumably very small.

According to this week Autosprint the problem with Ferrari tunnel is that it "lost calibration" over time.

I have been trying to imagine what could cause that. Assuming that the instruments are regularly calibrated and worn parts replaced, just creeping-sagging of the walls themselves comes to mind, but it seems quite a strong effect for displacements which are presumably very small.

Any ideas-experiences on the subject?

Adaptive walls are theoretically fine but I'm not sure many teams make a lot of use of them. They're complicated & can certainly lead to confusuion. The thinking is to keep as much as possible fixed to avoid any inconsistencies.Yes of course, all instrumentation must be regularly checked but that alone is not enough. How everything interacts, instruments, walls, rolling road functions, consistancy of air flow, quality of model, all add up to potential inconsistencies.Repeatability is everything. You need to be able to put the model back to a baseline configuration and acheive exactly the same result as previously. Anything different & you're just like dog chasing its tail.It can be a long, arduous & expensive process to get it right.

Adaptive walls are theoretically fine but I'm not sure many teams make a lot of use of them. They're complicated & can certainly lead to confusuion. The thinking is to keep as much as possible fixed to avoid any inconsistencies.Yes of course, all instrumentation must be regularly checked but that alone is not enough. How everything interacts, instruments, walls, rolling road functions, consistancy of air flow, quality of model, all add up to potential inconsistencies.Repeatability is everything. You need to be able to put the model back to a baseline configuration and acheive exactly the same result as previously. Anything different & you're just like dog chasing its tail.It can be a long, arduous & expensive process to get it right.

I was actually not referring to adaptive walls, just to structural, unwanted creep and sagging of normal walls and ceilings. I understand the concept of repeatability, but cannot fathom what could cause a lack of it in a wind tunnel that was previously OK, in the absence of technical failures.

I was actually not referring to adaptive walls, just to structural, unwanted creep and sagging of normal walls and ceilings. I understand the concept of repeatability, but cannot fathom what could cause a lack of it in a wind tunnel that was previously OK, in the absence of technical failures.

The less expensive 'open section' tunnels use air emitting from a 'bell' funnel, which passes across the model & rolling round & is gathered again by a similar funnel. This replicates (albeit not perfectly) a vehicle travelling in free air. The more commonly used 'closed section' tunnels have the air chanelled & re-cyclled completely in an enclosed 'donut' with the air being completley stabilised as it travels over the model & rr.There are formulae compensating for the 'closed wall' effect & as long as these give consistent results there is no great problem.

In both cases the air must be as consistent as possible & the air ducts are usually constructed of heavy steel plate with turning vanes, stiffening & sturdy concrete supports. They have to resist the force of a great mass of air travelling at speed, be resistant to vibration from the powerful electric drive motors and fans. It's hard to envisage that these structures, worthy of shipbuilding, can distort significantly, if properly designed but may be worth investigating if all else fails.

The rolling road along with the pitch, roll & yaw systems are complicated & intricately designed, installed by specialist companies & of course need consistent movements. This accounts for the several months of set up & calibration it takes to commission a tunnel.

Complicated business requiring patience & a good 'nose' for the problems.

Looks to be steel construction, hard to imagine any of the pieces being taken past their yield thresholds by use and dimensional stability being compromised. Seismic activity (not uncommon in Italy and there were significant recent tremors in the area) seems like a likely possible culprit.

Looks to be steel construction, hard to imagine any of the pieces being taken past their yield thresholds by use and dimensional stability being compromised. Seismic activity (not uncommon in Italy and there were significant recent tremors in the area) seems like a likely possible culprit.

Gosh, I should have recognized that as the Benetton/Enstone tunnel. Seems a shame it was never used as designed but I guess the logistics of running a pressurized tunnel outweighed the scaling/Reynolds number upsides. I've seen the Ferrari tunnel from the outside and it looks very impressive even from without. I wonder if it would be feasible to suspend an entire tunnel seismically like skyscrapers in Japan are and if that would do any good at all or just make routine calibration impossible.

That Benetton tunnel never operated in overpressure, although it was meant to. Boosting the pressure increases Reynolds number.

The problem with these tunnels is safety testing; I was told at the time by a person involved in these designs, that regulations require pressure testing of vessels (and the same regulation applies to wind tunnels, or applied at the time anyway) to be made by filling them with water and applying pressure to the water.
The idea is that in case of failure, there is no explosion as it would happen with a gas, just a leak.

Of course filling a wind tunnel with water is quite impractical; I guess the Benetton people thought they could get away with some different form of testing (after all it was just 1 atm), yet it didn't happen.

Shortly after it was built, just to make sure everything inside was OK. The spookies part of checking it from the inside is walking upwind of the working section, into the deepening gloom, and finding the floor suddenly dropping away. All that slippery gloss white paint and you have visions of yourself like a spider in a bath.

Shortly after it was built, just to make sure everything inside was OK. The spookies part of checking it from the inside is walking upwind of the working section, into the deepening gloom, and finding the floor suddenly dropping away. All that slippery gloss white paint and you have visions of yourself like a spider in a bath.

Yes - And I hope you have more sympathy towards spiders in baths now....

Looks to be steel construction, hard to imagine any of the pieces being taken past their yield thresholds by use and dimensional stability being compromised. Seismic activity (not uncommon in Italy and there were significant recent tremors in the area) seems like a likely possible culprit.

If in fact this was the problem, it should have triggered a calibration event. Then again, why wasn't there a routine periodic calibration check? Strange for equipment that is relied upon so heavily.Also, I wonder if they do the passenger car work in that tunnel. If they do, it's possible that they have bad results on that side. Haven't heard anything about that, though...

Doubt it. (a) that tunnel wasn't part of the recent Euro windtunnel round robin (b) why would a production car be tested in a 60% tunnel ? Blockage effects are discernible ven on full size tunnels with full size vehicles, so even if you could put a full size car in a tunnel designed for 60% it would give pretty horrible results.

Wouldn't 60% modeling work the same for production as F1 in a 60% tunnel? And conversely why do the F1/racing guys use 60% tunnels? I assume it's a cost constraint but surely making models is expensive as well?

Wouldn't 60% modeling work the same for production as F1 in a 60% tunnel? And conversely why do the F1/racing guys use 60% tunnels? I assume it's a cost constraint but surely making models is expensive as well?

Hopefully Paolo will pipe up. I was very interested to see plots of the six coefficients against speed, which of course we'd expect to be a horizontal line if the stories they tell us were true. I bet those curves are different if you use a different sized model. The issue is that some effects scale at different rates than others, so the F1 guys have to distort the shape of their 60% models to get the same interactions as the 100% vehicle.

Hopefully Paolo will pipe up. I was very interested to see plots of the six coefficients against speed, which of course we'd expect to be a horizontal line if the stories they tell us were true. I bet those curves are different if you use a different sized model. The issue is that some effects scale at different rates than others, so the F1 guys have to distort the shape of their 60% models to get the same interactions as the 100% vehicle.

Where'd you see the plots? The video linked to in the OP has been pulled off youtube. If the model designers are distorting the models, unless they understand 100% what they are doing (and if they do they should probably be designing the cars!) that it seems might introduce a dauntingly subjective variable to calibrating across different design iterations.

I saw them at a seminar on the results of the round-robin - something like 20 different cars tested in 10 different wind tunnels. They also tested the effects of some mods to one car in a few of the tunnels.